Actinic and Electron Beam Radiation Curable Water Based Electrode Binders and Electrodes Incorporating Same

ABSTRACT

A process for manufacturing an electrode utilizing electron beam (EB) or actinic radiation to cure the electrode binder is provided. A process is also disclosed for mixing specific actinic or EB radiation curable polymer precursors with electrode solid particles to form an aqueous mixture, application of the mixture to an electrode current collector, followed by the application of actinic or EB radiation to the current collector for curing the polymer, thereby binding the electrode binder to the current collector. Lithium ion batteries, electric double layer capacitors, and components produced therefrom are also provided.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. applicationSer. No. 14/279,963 having a filing date of May 16, 2014, which isincorporated herein by reference, and which claims filing benefit ofU.S. Provisional Patent Application Ser. No. 61/824,613 having a filingdate of May 17, 2013, which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.DE-EE0005421 awarded by the Department of Energy. The government hascertain rights in this invention.

FIELD OF THE DISCLOSURE

The present invention is in the technical field of electrodes as may beutilized in alkali-ion secondary (rechargeable) batteries, andparticularly in the technical field of lithium ion secondary batteries,and in electric double layer capacitors and manufacture thereof.

BACKGROUND

Electrochemical devices including batteries and electric double layercapacitors (EDLCs) have found great usefulness in power supplies,including power supplies of portable devices and auxiliary powersupplies for automobiles. For example, lithium ion batteries are one ofthe most popular battery types for use in portable electronics such asphones, music players, portable computers, and so forth. Lithium ionbatteries have very high energy-to-weight ratios, no memory effect, anda slow loss of charge when not in use. Lithium ion batteries are alsogrowing in popularity for military, electric vehicle, and aerospaceapplications due to their high energy density.

The basic working unit of a lithium ion battery is an electrochemicalcell. The electrochemical cell includes two electrodes, an anode andcathode that are physically separated from one another by a separatorand are ionically connected to each other by an electrolyte. The anodetypically includes a thin metal sheet of electrically conductingmaterial, such as copper, which is referred to as the anode currentcollector and is coated with solid anode material particles. The anodeparticles are held to the anode current collector and to each other byan anode binder, which is typically a polymer that retains adhesion andphysical characteristics such that it does not swell or disintegrateduring use. Typical anode particles include graphite, or silicon-basedmaterials or titanium materials or combinations of the three materialsand optionally with some carbon black. The sizes of the solid anodeparticles coated on the anode current collector typically range fromseveral nanometers to tens of microns in nominal diameter.

The lithium ion battery electrolyte may be liquid, solid or a gel. Forliquid electrolytes, a separator is employed to maintain the physicalseparation of the anode from the cathode. A typical separator is a thinporous polymer sheet in which the void spaces are filled withelectrolyte. A typical liquid electrolyte is a mixture of organiccarbonates such as alkyl carbonate containing complexes of lithium ions,generally non-coordinated anion salts such as lithiumhexafluorophosphate (LiPF₆), lithium hexafluoroarsenate monohydrate(LiAsF₆), lithium perchlorate (LiClO₄), lithium tetrafluoroborate(LiBF₄), and lithium triflate (LiCF₃SO₃). Typical solid electrolytes arepolymers. For instance, U.S. Pat. No. 7,387,851 to Gozdz, et al.describes an electrolyte formed from a liquid polymer, oligomer, ormonomer electrolyte precursor that can be crosslinked to form a solidpolymer electrolyte. A wide variety of materials may be used as a gelelectrolyte. The electrolytes are designed to withstand the voltagebetween the anode and the cathode, and offer a high mobility of lithiumions without a risk of flammability.

The cathode typically employed in a lithium ion battery includes a thinmetal sheet of electrically conducting material such as aluminum, whichis referred to as the cathode current collector, and is coated withsolid cathode particles. Cathode particles are held to the cathodecurrent collector and to each other by a cathode binder, which like theanode binder is typically a polymer that retains adhesion and physicalcharacteristics such that it does not swell or disintegrate during use,and the cathode binder and anode binder can be the same or can differfrom one another. Typical cathode particles include lithium metal oxidessuch as lithium cobalt, manganese, nickel, or vanadium oxides, and otherlithium compounds such as lithium iron phosphate. The cathode particlesoften include a small amount of carbon as well, to improve conductivity,though the carbon will generally not be as graphitic as the main carbonof the anode. The sizes of the cathode particles coated on the currentcollector range from several nanometers to several microns in nominaldiameter.

An EDLC, also known as a supercapacitor or an ultracapacitor, is anelectrochemical capacitor that has an unusually high energy density whencompared to traditional capacitors. An EDLC includes two separateelectrodes of the same construction separated by an interveningsubstance that provides effective separation of charge despite avanishingly thin (on the order of nanometers) physical separation of thelayers. The electrode of an EDLC employs a current collector, typicallya current collector similar to that of a lithium ion battery cathode,such as aluminum. To improve energy storage density a nanoporousmaterial, a porous particulate carbon such as graphite or activatedcharcoal is applied to the surface of the current collector with abinder, which is typically a polymer produced to retain adhesion andhardness and not swell or disintegrate during use. The particle size ofthe carbon generally ranges from several nanometers to several micronsin nominal diameter. The pores of the carbon are filled with theintervening substance, i.e., an electrolyte that is a liquid or a gel. Atypical liquid electrolyte is an organic alkyl carbonate that caninclude selected lithium salts.

A typical process for forming an electrode such as is found in a lithiumion battery or an EDLC includes:

-   -   1) The polymeric binding material is formed into a solution with        a solvent such that the solution has a suitably low viscosity        for application to the current collector after mixing with the        solid particles.    -   2) The low viscosity binding solution is mixed with the        electrode solid particles at approximately 20-80 wt. % of the        solvent, and particularly approximately 50 wt. % of the solvent        to form a paste.    -   3) The paste is coated in a thin layer (typically 10 to 200        microns) onto the current collector using conventional coating        techniques.    -   4) The coated current collector is passed through a thermal        drying oven where solvent is driven off and the binder polymer        is set.    -   5) The electrode is passed through a pair of rotating rollers        separated by a narrow gap (e.g., 5 to 200 microns) to compress        the current collector coating to a specified thickness.    -   6) Typically, both sides of the electrode current collector are        coated with the solid anode or cathode particles and processed        by the aforementioned steps.

There are multiple shortcomings involved in the manufacturing ofelectrodes that have a direct effect on the cost of manufacturing. Theseshortcomings include, without limitation:

-   -   a) Solvent used to dissolve the polymer binding material must be        vaporized requiring substantial thermal energy input.    -   b) There are substantial energy inefficiencies associated with        thermal drying.    -   c) The vaporized solvent must be recovered and either disposed        of or recycled.    -   d) The oven required for drying the polymer binding material        occupies significant manufacturing space at a significant        capital cost.    -   e) The time required to manufacture the electrodes is increased        by the time required for the polymer binding material to be        dried in the drying oven.

What are needed in the art are improved materials and methods forforming electrodes. For instance, improved electrode binders for use inlithium ion cathodes and anodes and EDLC electrodes would be of greatuse.

SUMMARY

According to one embodiment, a method of forming an electrode isdisclosed. In general, a method can include combining a polymerprecursor, water, and solid electrode particles to form an aqueousmixture. The aqueous mixture can also include a crosslinking agentcapable of forming covalent bonds with the polymer precursor uponsubjection of the mixture to actinic or electron beam radiation. Thepolymer precursor can be dispersed or dissolved in the water of theaqueous mixture.

The method also includes applying the aqueous mixture to a surface of acurrent collector to form a layer, and thereafter subjecting the layerto actinic or electron beam radiation, thereby polymerizing andcrosslinking the polymer precursor to form an electrode binder thatserves to bind the solid electrode particles to each other and thecurrent collector and form the electrode. Electrodes described hereincan encompass both cathodes and anodes.

Also disclosed is an electrode that can be formed according to themethod. For instance, an electrode can include a current collector and alayer on the current collector that includes solid electrode particlesheld in an electrode binder, the electrode binder including crosslinkedpolymer precursors. The electrode binder can also include a reactedactinic radiation or electron beam curable crosslinking agent covalentlybonded to the polymer precursors. In one embodiment, the electrodebinder can also include a photoinitiator.

Also disclosed are products incorporating the electrodes such as abattery, e.g., a lithium ion battery, or a double layer capacitor. Ingeneral, the product can include an electrode including the electrodebinder held adjacent to one or more additional components of the devicesuch as a second electrode, a separator, an electrolyte, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of one embodiment of an electrode manufacturingprocess as disclosed herein.

FIG. 2 is a plan view of another embodiment of an electrodemanufacturing process as disclosed herein.

FIG. 3 is a cross-sectional view of a lithium ion electrochemical cellaccording to one embodiment of the disclosure.

FIG. 4 is a cross-sectional view of an EDLC according to one embodimentof the disclosure.

FIG. 5A and FIG. 5B present testing results for a cell including anelectrode binder as described herein.

FIG. 6A and FIG. 6B present testing results for a cell including anelectrode binder as described herein.

DETAILED DESCRIPTION

Reference now will be made in detail to various embodiments of thedisclosed subject matter, one or more examples of which are set forthbelow. Each example is provided by way of explanation, not limitation,of the subject matter. In fact, it will be apparent to those skilled inthe art that various modifications and variations may be made in thepresent disclosure without departing from the scope or spirit of thedisclosure. For instance, features illustrated or described as part ofone embodiment, may be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present disclosurecover such modifications and variations.

In general, the present disclosure is directed to a process formanufacturing electrodes without the need for the costly oven-drying orsolvent handling. Also disclosed are electrodes formed according to theprocess and products such as lithium ion batteries, EDLCs and others asmay incorporate the electrodes. More specifically, disclosed electrodesincorporate water dispersible or dissolvable polymer precursors curedwith actinic radiation or electron beam radiation.

The process allows for the formation of the electrodes by use of anaqueous mixture in which the polymer precursors are dispersed ordissolved in water in conjunction with solid electrode particles.Utilization of an aqueous mixture in forming the electrodes can providemultiple benefits to the process. For instance, this can decrease costsassociated with the formation of the electrodes both directly, as largeamounts of expensive organic solvents need no longer be obtained for theprocess, and indirectly, as recovery processes for organic solvents canbe limited or avoided all together. For instance, the aqueous mixtureused to form the electrode binders can include less than about 5 wt. %,or about 1 wt. % to about 2 wt. % organic solvents by weight of themixture. Moreover, the formation process can be cleaner, with theproduction of little or no potentially dangerous waste products. Uponcure via electron beam or actinic radiation, the waterdispersible/dissolvable polymer precursors demonstrate good adhesion tocurrent collectors (e.g., copper or aluminum) while providing excellentresistance to the harsh operating conditions and electrolytic materialpresent in both batteries and EDLCs.

As utilized herein, the term actinic radiation is intended to refer toelectromagnetic radiation that is capable of producing photochemicaleffects. For instance, the aqueous mixture including the polymerprecursors can be cured by actinic radiation in the ultraviolet orvisible spectrum, both of which can encompass actinic radiation.Conventional ultraviolet and electron beam curable binder resins haveonly recently been utilized successfully in electrode manufacturing tocure (i.e. polymerize and crosslink) electron binders (see, U.SPublished U.S. Patent Application No. 2011/0081575 to Voelker, et al.,which is incorporated herein by reference). In the past, electron beamand actinic radiation curing was limited to the formation of thickopaque films that would not hold up well in the harsh environment of abattery or EDLC. The aqueous mixtures described herein include polymerprecursors rather than large fully formed polymers as have been used toform electron binders in the past. Upon curing, the polymer precursorsform electron binders that can adhere well to the current collector andalso maintain integrity in the harsh environment of the final product.

The polymer precursors can include monomers and/or oligomers thatinclude functional groups curable by use of actinic or electron beamradiation. Curable functional groups are generally selected fromcarbon-carbon double bond, thiol and epoxy groups. By way of example,carbon-carbon double bond containing curable functional groups include(meth)acrylate, (meth)acrylamide, (meth)acrylthioester, N-vinylamide,vinylester, vinylthioester, allylether, allyamine, allylsulfide,N-vinylamine, vinylether, vinylsulfide, maleate, fumarate, maleamide,fumaramide, thiomaleate, thiofumarate, maleimide, citraconimide andnorbornene groups.

In one embodiment, the polymer precursors can include water dispersibleor dissolvable monomers including, without limitation, methyl acrylate,ethyl acrylate, 2-chloroethyl vinyl ether, 2-ethylhexyl acrylate,hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, ormixtures thereof. The polymer precursors can be solely monomers that canbe crosslinked to form the crosslinked network of the electrode binder.Alternatively, monomeric polymer precursors can be utilized inconjunction with oligomeric polymer precursors. The use of monomericpolymer precursors is not a requirement, however, and only oligomericpolymer precursors can be utilized in one embodiment and the oligomericpolymer precursors can be crosslinked by use of a crosslinking agent inconjunction with the application of actinic or electron beam radiation.

The polymer precursors can generally have a number average molecularweight of less than about 150,000. For instance, when consideringoligomeric polymer precursors, the oligomers can have a number averagemolecular weight of from about 7,000 to about 150,000, from about 10,000to about 130,000, from about 10,000 to about 50,000, or from about15,000 to about 40,000.

Oligomers that can be utilized as polymer precursors are notparticularly limited, provided that they are or can be functionalized soas to be water dispersible or dissolvable and include suitablecrosslinking functionality as described. For example, functionalizedpolyurethanes, polyesters, polyolefins, cellulose derivatives, rubbers,latex, polysulfides, acrylics, silicones, and so forth can be utilized.

In one embodiment, the polymer precursor can be a functionalizedpolyurethane precursor. Functionalized polyurethane precursors can bepurchased or formed, as desired. For instance, suitable functionalizedpolyurethane precursors are available from Cytec Industries Inc.Functionalized polyurethane precursors can also be formed according toknown methodology, for instance as described in U.S. Pat. No. 8,431,673to Wang, et al.

The functionalized polyurethane precursors can include the reactionproduct of a soft segment component, an isocyanate, and a crosslinkingcomponent that can provide the desired functionality. The functionalizedpolyurethane can be formed of any suitable di-isocyanate and anysuitable soft segment. For example, the di-isocyanate can be aromatic,aliphatic, araliphatic or cycloaliphatic polyisocyanate or derivativesthereof containing urethane, isocyanurate, allophanate, biuret,uretdione and/or imino-oxadiazinedione groups. Examples of suitablepolyisocyanates can include, without limitation,diphenylmethane-2,4′-diisocyanate, diphenylmethane-4,4′-diisocyanate,2,4-toluene-diisocyanate, 2,6-toluene-diisocyanate,1,6-diisocyanatohexane, dicyclohexylmethane-2,4′-diisocyanate,dicyclohexylmethane-4,4′-diisocyanate,5-isocyanato-1-isocyanatomethyl-1,3,3-trimethylcyclohexane (isophoronediisocyanate, IPDI), meta-tetramethylxylylene diisocyanate,1,3-bis-(isocyanatomethyl)benzene, 1,5-naphthylene diisocyanate, theirdi-, tri- or oligomers and their adducts with polyols.

In one embodiment, the soft segment molecule can be a diol. In addition,the soft segment can include any linking segments along the soft segmentbackbone as is generally known in the art. For example, in variousembodiments, the soft segment can include polycarbonate, dimer acid,polyester, or polyether linking segments. Examples of the soft segmentcan include, without limitation, 2-phenoxyethanol,3-(methylthio)-1-propanol, 4-(methylthio) benzyl alcohol and2-phenylthioethanol.

The crosslinking component can include a first functional group that canreact with the soft segment or the isocyanate and can also include asecond functional group that is curable by actinic or electron beamradiation. The first functional group can include, for example, analcohol, a thiol, or an amine. The second functional group can includecurable functional groups as described above. Examples of thecrosslinking component can include, without limitation, (meth)acrylatesand (meth)acrylamides, such as 2-hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, 4-hydroxybutyl acrylate, hydroxypropyl acrylate (2isomers), hydroxypropyl methacrylate, pentaerythritol triacrylate,di-tri methylolpropane triacrylate, tricyclodecane-dimethanolmonoacrylate, N-methylethanolamine acrylate,N-(hydroxymethyl)acrylamide; N-vinylamides such as5-(hydroxymethyl)-1-vinyl-2-pyrrolidinone, 3-hydroxy-N-vinylproprionamide (2-cyano-N-vinylacetamide hydrolyzed to carboxylic acid,then reduced to alcohol); allyls such as diallylamine, allyl alcohol,N-allyl-N-methylamine, N-allyl-N-cyclopentylamine, N-allyl-N-phenylamine, N-allyl-2,2′-iminodiethanol; vinylethers such as 4-hydroxybutylvinylether, ethylenegylcol monovinylether,3,4-dihydro-2H-pyran-2-methanol, diethyleneglycol monovinylether;norbornenes such as 5-norbornene-2-methanol, 5-norbornene-2,3-dimethanolmonoacrylate; epoxides such as 2-oxiranylmethanol,(2S,3S)-trans-3-phenyloxirane-2-methanol, trimethylolpropanediglycidylether, pentaerythritol triglycidylether, Bisphenol-Amonoglycidylether, Bisphenol-F monoglycidylether, Bisphenol-A(3-chloro-2-hydroxypropyl) glycidylether.

A functionalized polyurethane can generally be prepared according to atwo-step process. For instance, in the first step, at least part of thepolyisocyanate can be reacted with the crosslinking compound, and in asecond step the reaction product obtained is further reacted with thesoft segment. The reaction can be performed in the presence of acatalyst, such as for example amine and orgametallic complex catalystssuch as dibutyl tin dilaurate. The reaction can be performed at atemperature of from about 20° C. to about 120° C., and can be carriedout in the presence of a polymerization inhibitor or stabilizer or anycombination thereof. Primary anti-oxidants (free-radical inhibitors)such as quinones, in particular hydroquinone, and di-tert butyl p-cresolcan be utilized.

The polymer precursor can be a functionalized polyolefin that is waterdispersible or water dissolvable. For instance, oligomeric polyethylene,polypropylene, or copolymers thereof can be polymer precursors of theelectrode formation process. The polyolefin polymer precursor can bearone or more substituents, e.g., a functional group such as a carbonyl,sulfide, acrylate, etc. to encourage crosslinking during formation ofthe electrode. As utilized herein, the term “olefin” generally refers toaliphatic, alicyclic or aromatic compounds having one or more doublebonds. Representative olefins include ethylene, propylene, 1-butene,1-hexene, 1-octene, 4-methyl-1-pentene, butadiene, cyclohexene,dicyclopentadiene, styrene, toluene, α-methylstyrene, etc. A variety ofpolyolefin polymers can be used including, without limitation,polyethylene, polypropylene, ethylene/styrene interpolymers (ESI), andcatalytically modified polymers (CMP), e.g., partially or fullyhydrogenated polystyrene or styrene/butadiene/styrene block copolymers,polyvinylcyclohexane, etc.

Cellulose derivatives can be utilized in forming the electrodes. Forinstance, partially or completely alkylated or acrylated cellulose canbe utilized. In one embodiment, cellulose acrylate that is a fatty acidester of cellulose obtained by substituting a hydroxyl group ofcellulose with an acetyl group or an acyl group having 3 or more carbonatoms can be utilized as a polymer precursor. For instance, the degreeof substitution of the hydroxyl group of cellulose can satisfy thenumerical formula:

1.0<A+B<3.5

in which A is the degree of substitution of hydroxyl groups by an acetylgroup, and B is the degree of substitution of hydroxyl groups by an acylgroup having 3 or more carbon atoms.

The acyl group having 3 or more carbon atoms may be an aliphatic groupor an aromatic hydrocarbon group. For instance, the acyl group having 3or more carbon atoms may be an alkylcarbonyl ester of cellulose,alkenylcarbonyl ester or aromatic carbonyl ester, and aromaticalkylcarbonyl ester, and they may be further substituted. Specificexamples include, without limitation, a propionyl group, a butanoylgroup, a heptanoyl group, a hexanoyl group, an octanoyl group, adecanoyl group, a dodecanoyl group, a tridecanoyl group, a tetradecanoylgroup, a hexadecanoyl group, an octadecanoyl group, an isobutanoylgroup, a t-butanoyl group, a cyclohexanecarbonyl group, an oleoyl group,a benzoyl group, a naphthylcarbonyl group, a cinnamoyl group, and thelike. Among these, preferred are a propionyl group, a butanoyl group, adodecanoyl group, an octadecanoyl group, t-butanoyl group, an oleoylgroup, a benzoyl group, a naphthylcarbonyl group, a cinnamoyl group, andthe like.

Specific examples of acrylated cellulose encompassed herein include,without limitation, cellulose acetate, cellulose acetate propionate,cellulose acetate butyrate, and mixtures thereof.

Derivatives of cellulose for use as polymer precursors can be formedaccording to methods as are generally known in the art. A typicalsynthesis method is a liquid phase acetylation method with the use of acarboxylic anhydride/acetic acid/sulfuric acid catalyst as described inU.S. Pat. No. 7,951,430 to Sugiyama, et al., which is incorporatedherein by reference. The formation method can include separating highmolecular weight constituents from the product and utilizing the lowmolecular weight constituents as polymer precursors in an electrodeformation method.

In one embodiment, the polymer precursors can include rubber or latexoligomers. For example, polyisoprene and/or polybutadiene and/orstyrenebutadiene rubbers as described in U.S. Published PatentApplication Serial No. 2011/0081575 to Voelker, et al., previouslyincorporated by reference, can be utilized. However, the rubber/latexpolymer precursors are not limited to materials incorporating onlyisoprene and/or butadiene components. Functionalized rubber polymerprecursors as may be utilized can include at least one of isoprene,butadiene, cyclopentadiene, ethylidene norbornene, and vinyl norbornenemonomer units, or combinations thereof.

One embodiment of a polymer precursor is a carboxylated methacrylatedisoprene backbone having the general formula:

wherein m is between about 10 and about 1000, or between about 100 andabout 1000, or between about 200 and about 500; and n is between 1 andabout 20, or between 1 and about 10, or between about 2 and about 10, orbetween about 2 and about 5.

Another embodiment of a polymer precursor can incorporate a carboxylatedmethacrylated butadiene backbone and can have the general formula:

wherein m is between about 10 and about 1000, or between about 100 andabout 1000, or between about 200 and about 500; and n is between 1 andabout 20, or between 1 and about 10, or between about 2 and about 10, orbetween about 2 and about 5.

Yet another rubber polymer precursor can incorporate a butadienebackbone and can have the general formula:

wherein n is between about 5 and about 2000, or between about 10 andabout 1500, or between about 100 and about 1000.

Of course, the polymer precursor can include multiple different backbonesegments. For instance isoprene-butadiene copolymers can be utilized aspolymer precursors.

The polymer precursors can be functionalized to include additionalacidic reactive groups that improve water dispersability, metal adhesionand/or improve curability by EB or actinic radiation crosslinking. Forinstance, polymer precursors can be functionalized with carboxy, amino,and/or hydroxyl functionality that can increase hydrophiliccharacteristics of the polymer precursors and improve dispersability ofthe polymer precursors in the aqueous mixture.

In general, the polymer precursor can be included in an aqueous mixturein an amount from about 4 wt. % to about 25 wt. %, from about 5 wt. % toabout 20 wt. %, or from about 6 wt. % to about 18 wt. % of the aqueousmixture.

In addition to the polymer precursor, the aqueous mixture can includesolid electrode particles. The solid electrode particles can include anyparticulate materials as are generally known in the art such as, withoutlimitation, carbon particulate materials such as graphene, activatedcarbon, graphite, low sulfur graphite, carbon black, carbon nanotubes,etc.; ceramic materials including silicon-based materials such assilicon oxides; metal oxide salts such as oxides of lithium, cobalt,manganese, nickel, aluminum or vanadium; and so forth. By way ofexample, particulate materials can include lithium compounds (i.e.,lithiated compounds) such as lithium manganese oxide, lithium cobaltoxide, lithium nickel oxide, lithium nickel cobalt aluminum oxide,lithium iron phosphate, lithium nickel manganese cobalt (NMC), as wellas mixtures thereof. The preferred materials for any embodiment can varydepending upon the type of electrode (cathode or anode) as well as theproduct that will incorporate the electrode, as is known.

The aqueous mixture can generally include the solid electrode particlesin an amount of from about 50% by weight of the mixture to about 90% ofthe mixture, or from about 55% by weight of the mixture to about 85% ofthe mixture.

The aqueous mixture can also include one or more crosslinking agents. Ofcourse, in those embodiments in which the polymer precursors includefunctionality to provide direct crosslinking among the precursors, theinclusion of a crosslinking agent may not be necessary. For instance,the aqueous mixture can include one or more crosslinking agents in anamount of from about 0.2 wt. % to about 1 wt. % of the composition, orfrom about 0.3 wt. % to about 0.7 wt. %. Exemplary crosslinking agentscan include those that can react with the polymer precursors of themixture when subjected to EB and/or actinic radiation. Specificradiation suitable for each crosslinker is generally known in the art.For instance, a crosslinker can react upon subjection of the mixture toactinic radiation in the UV spectrum or in the visible spectrum.Examples of crosslinking agents can include, without limitation,monofunctional acrylates, difunctional acrylates, and multifunctionalacrylates and other vinyl compounds. Suitable acrylates may be linear,branched, cyclic, or aromatic. Linear acrylates can include alkylacrylates wherein the alkyl contains from 4 to 20 carbon atoms. Branchedacrylates can include branched alkyl acrylates wherein the alkylcontains from 4 to 20 carbon atoms such as 2-ethylhexyl acrylate orisostearyl acrylate. Cyclic acrylates can include dicyclopentanylacrylate and n-vinyl caprolactam. Aromatic acrylates can includephenoxyethylacrylate. Difunctional and multifunctional acrylates caninclude 1,6-hexandiodi(meth)acrylate, 1,9-hexandiodi(meth)acrylate, andtricyclodecanedimethanol diacrylate.

In those embodiments in which the polymer precursors are cured in thepresence of actinic radiation, the aqueous mixture can also include aphotoinitiator. A photoinitiator may be present in the aqueous mixtureat concentrations up to about 1 wt. %, for instance from about 0.2 wt. %to about 1 wt. %, or from about 0.3 wt. % to about 0.7 wt. % of theaqueous mixture.

Exemplary photoinitiators can include, without limitation, benzophenone,hydroxyacetophenone, methylbenzophenone, 4-Phenylbenzophenone,4,4′-Bis(diethyl amino)benzophenone, Michler's Ketone,4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-methylpropyl)ketone, and otherbenzophenone derivatives, benzyldimethyl ketal, 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1 butanone;2-mercaptobenzoxazole, camphorquinone,2-hydroxy-2-methyl-1-(4-t-butyl)phenlypropan-1-none,2-methyl-1-[4-(methylthiophenyl)-2-morholinopropanone, maleimides,2,4,5-trimethylbenzoly-diphenyl phosphine oxides,bis(2,6-dimethyloxybenzoyl) 2,4,4-trimethylpentyl) phosphine oxide,bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide, polymericphotoinitiators derived from the above, and combinations thereof. In oneembodiment, a propanone photoinitiator may be utilized such as a blendof about 70 wt. %oligo(2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone and about30 wt. % 2-hydroxy-2-methyl-1-phenyl propan-1-one, commerciallyavailable from Lamberti USA, Inc., Conshohocken, Pa. under the tradename Esacure® KIP 150 or KIP 100F. Other photoinitiators sold by Lamberti USA, Inc. under the KIP or Esacure® designation may also beutilized, such as Esacure SM 303. Other polymeric photoinitiatorsinclude PL-816A from Palermo Lundahl Industries. In another embodiment,an oxide photoinitiator may be utilized. One suitable oxidephotoinitiator is bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxidecommercially available from Ciba Specialty Chemicals, Tarrytown, N.Y.under the trade name Irgacure® 819. Other photoinitiators sold by CibaSpecialty Chemicals under the Irgacure® trade name are also suitable foruse.

Certain compounds act as both a photoinitiator and a crosslinker. Thesecompounds are characterized by the ability to generate two or morereactive species (e.g., free radicals, carbenes, nitrenes, etc.) uponexposure to UV-light and to subsequently covalently bond with twoprecursor polymers. Any compound that can perform these two functionscan be utilized, and representative compounds include sulfonyl azidesdescribed in U.S. Pat. No. 6,211,302 to Ho, et al. and U.S. Pat. No.6,284,842 to Ho, et al., both of which are incorporated herein byreference.

The aqueous mixture may include other additives suitable for the desiredproperties of the mixture or the cured electrode binder. Additives maybe utilized in the coating composition up to about 10 wt. % for eachadditive, in some embodiments up to about 5 wt. %, and in someembodiments up to about 2 wt. % of the aqueous mixture. Additives thatmay be incorporated in the aqueous mixture can include, withoutlimitation, coupling agents, adhesion promoters, dispersants, curingaccelerants, photosensitizers, wetting agents, defoamers, etc. Forexample, a suitable coupling agent is γ-glycidoxypropyltrimethoxysilanesuch as Silquest® A-187, commercially available from MomentivePerformance Materials, Albany, N.Y.

In one embodiment, a wetting agent can be included in the aqueousmixture. A wetting agent can improve the contact and wetting between thesolid electrode particles, the polymer precursor, and the currentcollector onto which the aqueous mixture can be applied during formationof the electrode. Accordingly, inclusion of a wetting agent can improvethe adherence between the various components following cure of theelectrode binder. A wetting agent can also improve the solubility anddispersability of the other components of the aqueous mixture.

Wetting agents can include both sacrificial materials, which willgenerally be volatized prior to or during the cure of the binder coatingcomposition, as well as materials that can remain in the productfollowing cure. For instance, a wetting agent can also function as anelectrolyte following cure of the binder. Exemplary wetting agents caninclude, without limitation, acetone, isopropyl alcohol, dimethylcarbonate, and the like. In general, any solvent or electrolyte materialthat can improve wetting and contact between the components of theaqueous mixture and the current collector can be utilized. In oneembodiment, fast evaporating, low boiling temperature wetting agents canbe utilized. By way of example, a wetting agent can have a boiling pointof less than about 160° F. (about 71° C.). Beneficially, by utilizationof a low boiling point wetting agent, the wetting agent can bedissipated during the UV/EB cure, and the substantial thermal energyinput necessary for solvent removal of previously known processes is notnecessary. Alternatively, wetting agents can be utilized that aredesigned to remain in the material following the cure, for instance foruse as an electrolyte.

Referring now to the figures, FIG. 1 and FIG. 2 present embodiments forapplying the aqueous mixture as a layer 5 to an electrode currentcollector 2 and FIG. 3 illustrates a formed product. In the FIG. 3, thesolid electrode particles 9 and the remainder of the aqueous mixture 10,are shown as separate layers. In general, however, the two are appliedto a current collector 2 pre-mixed as a single aqueous mixture to form asingle layer 5. The polymer precursors of the layer 5 are cured on thecurrent collector 2 utilizing actinic and/or EB radiation. Followingcrosslinking to form a matrix adhered to a current collector, thecrosslinked polymer precursors can exhibit excellent chemical resistanceand can be insoluble in electrolytes at elevated temperatures whiledemonstrating exceptional adhesion to current collectors.

In further detail, referring to FIG. 1 and FIG. 2, an electrode currentcollector feed roll 1 supplies electrode current collector 2. Anapplicator 3 can mix the solid electrode particles with the remainder ofthe aqueous mixture and apply a thin layer 5 of the aqueous mixture tomoving current collector 2. This application coating may be accomplishedby conventional coating techniques such as, gravure, flexo, slot die,reverse roll, knife over roll, flat and rotary screen printing,extrusion, offset, or the like.

Following formation of the electrode layer 5, the layer can be subjectedto actinic radiation 4 and/or EB radiation 8, which can crosslink thepolymer precursors of the layer 5. For instance, upon subjection of theaqueous mixture to UV, visible and/or EB radiation and, when necessary,in the presence of a photoinitiator, the crosslinking agents of theaqueous mixture can react with reactive functional groups of the polymerprecursors, forming covalent bonds throughout the layer and therebyfirmly encapsulating the solid electrode particles within thecrosslinked network and also firmly binding the layer 5 to the currentcollector 2.

The formation method can incorporate a relatively short residence timefor actinic radiation curing 4 and/or EB curing 8. The resultingapplication of the layer 5 to electrode current collector 2 and thecrosslinking thereof can thus increase production speeds and reducecosts. Multiple applicator stations 3 may be employed to build upseveral layers of electrode coating materials, optionally with separatorlayers therebetween, so that the resulting final thickness required canbe accomplished at high speeds of for example from about 20 ft/min toabout 400 ft/min. Separators that can be included between layers ofelectrodes can be any separator as is generally known in the art. Forinstance, when forming an EDLC or a lithium ion battery, a separator canbe applied between adjacent electrode layers that is formed of a porous(and often reinforced) polymeric sheet, such as polytetrafluoroethylene(PTFE), polypropylene (PP), polyethylene (PE), polyethyleneterephthalate (PET), or fused layers of PP & PE, and the like.

The aqueous mixture is generally applied in a very thin layer 5 toelectrode current collector 2. Electrode layer 5 thickness may be fromabout 1 micron to about 500 microns, from about 5 microns to about 250microns, from about 5 microns to about 200 microns, or from about 5microns to about 150 microns. A layer 5 may be applied to one or bothsides of the current collector 2. FIGS. 1 and 2 illustrate a systemapplying electrode layer 5 to each side of a current collector 2.

FIGS. 1 and 2 depict systems utilizing both actinic and EB radiationcuring 4 and 8, respectively. Depending on the characteristics of theaqueous mixture, actinic radiation device 4 alone, EB radiation device 8alone, or both may be utilized.

Referring to FIG. 1, an electrolyte 6 may be integrated with electrodecurrent collector 2 and layer 5. Electrolyte 6 can be a solid, a liquid,or a gel, as is known in the art. For example, electrolyte 6 can be anorganic electrolyte, such as a carbonate (e.g., ethylene carbonate ordiethyl carbonate containing complexes of lithium ions), or an aqueouselectrolyte, such as potassium hydroxide, sulfuric acid, or a liquidmixture of organic carbonates such as alkyl carbonate containingcomplexes of lithium ions (e.g., non-coordinated anion salts such asLiPF6, LiAsF6, LiClO4, LiBF4, and LiCF3SO3). If electrolyte 6 is liquid,a polymeric separator may be included in electrolyte layer 6. Generally,if electrolyte 6 is a solid or gel, an electrolyte separator is notneeded, though a separator may still be utilized in these embodiments.If layer 5 is applied to both sides of the current collector 2,electrolyte 6 may be integrated on each side of current collector 2. Theproduct may then be passed through calendar rolls 7 which can compressthe layers to a desired thickness. If required, electron beam radiationdevice 8 may radiate through the electrolyte 6 to cure the bindingmaterial.

Referring to FIG. 2, illustrated is a process for manufacturing anelectrode that does not incorporate an electrolyte. The techniquedepicted in FIG. 2 could be combined with the technique depicted in FIG.1 to assemble an electrochemical cell 11 such as shown in FIG. 3. Forexample, the process depicted in FIG. 1 may be utilized to construct theanode or cathode (current collector 2 and layer 5) and electrolyte 6.The process depicted in FIG. 2 may be utilized to construct the opposingelectrode without electrolyte 6. The products of FIG. 1 and FIG. 2 maythen be combined to construct an electrochemical cell.

For instance, FIG. 3 generally illustrates a lithium ion electrochemicalcell 11. As illustrated, the cell 11 includes current collector 2 with alayer 5 disposed on each side. The layer 5 includes anode (−) or cathode(+) active solid electrode particles 9 and the rest of the aqueousmixture 10. FIG. 3 also illustrates the solid electrode particles 9 andthe rest of the aqueous mixture 10 separate within layer 5 forconvenience of illustration. Electrolyte 6 and optionally an electrolyteseparator (not shown) may be disposed on each electrode layer 5. As oneskilled in the art appreciates, a lithium ion battery may comprise anynumber of electrochemical cells 11 in series or parallel as desired. Inaddition to cell 11, a lithium ion battery constructed in accordancewith the disclosure may further include insulation material, casings,control circuitry, connectors, etc. as will be appreciated by thoseskilled in the art. Furthermore, the battery can be any type of lithiumion battery such as cylindrical, prismatic, pouch-type, or otherbatteries as are understood in the art.

Similarly, a first electrode and a second identical electrode can beassembled with a suitable electrolyte and separator therebetween toconstruct an EDLC. For instance, and with reference to FIG. 4, an EDLC40 can include a first aluminum current collector 42 and a secondaluminum current collector 43. The first and second current collectors42, 43 can be separated by a separator 46. A first layer 44 and a secondlayer 45 on either side of the separator 46 can be the same ordifferent. For instance, both layer 44 and 45 can include an actinic/EBradiation cured aqueous mixture 50 that includes solid particulates 52,e.g. graphite, in the mixture. The separator 46 can be any standardseparator, for instance a porous PP-PE-PP film.

The present disclosure can provide numerous advantages. For instance,disclosed methods may significantly reduce manufacturing costs forelectrodes, and thereby products produced therefrom. The advantages ofthe present disclosure can include, without limitation:

-   -   a) Substantial reduction of processing time for curing electrode        binding materials.    -   b) Significant reduction in capital and operating costs by        eliminating the need for thermal curing ovens and the associated        energy inefficiencies of thermal drying in lieu of actinic        and/or EB radiation curing stations.    -   c) Substantial reduction in space, building, and infrastructure        and maintenance that accompany thermal curing. For example,        existing thermal lines for automobile batteries are 300 ft long        and run 10-20 ft per minute; two UV lamps can fit in a length of        2 feet (replacing 298 ft of production line) and produce        batteries at 200 ft/min. So to expand the thermal line to run at        the 200 ft/min, the thermal section of the production line would        have to be increased to at least 3,000 ft long or building needs        to be an additional 0.57 miles long.    -   d) Substantial reduction in energy and capital associated with        the dehumidification of the production area as these coatings        are usually applied in a clean room    -   e) The use of a shorter process line also means that foil does        not have to be as thick to withstand the tension and its own        weight during the long drying process; thinner foil results in a        lighter weight battery, which is extremely beneficial in vehicle        applications where the battery energy is used to move its own        weight as well as the weight of the vehicle    -   f) Substantial reduction or elimination of the requirement of        organic solvents which may substantially reduce or eliminate the        costs of Volatile Organic Compound (VOC) procurement, recovery,        and disposal.

The present invention may be better understood with reference to thefollowing examples.

Materials

Materials utilized in the Examples included the following:

Polymer Precursor

-   -   PP 1—Ucecoat® 7689 acrylated polyurethane dispersion available        from Cytec Industries, Inc.    -   PP 2—Ucecoat® 7699 aliphatic polyurethane dispersion available        from Cytec Industries, Inc.

Photoinitiator

-   -   PI 1—2-hydroxy-methyl-1-phenyl propanone    -   PI 2—bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide    -   PI 3—1-hydroxycyclohexyl phenyl ketone    -   PI 4—benzophenone

Solid Electrode Particles

-   -   SEP1—mesocarbon microbead graphite (MSMB)    -   SEP2—carbon black    -   SEP3—lithium (nickel cobalt manganese) oxide

Dispersant

-   -   Disp1—Tamol™ 1254—polyacid dispersant available from the Dow        Chemical Co.    -   Disp 2—Solsperse® 41000 available from Lubrizol Corp.

Accelerant

-   -   Triethanolamine

Photosensitizer

-   -   Thioxanthone and isopropanol mixture

Defoamer

-   -   Def1—Additol® VXW available from Cytec Industries, Inc.    -   Def2—BYK®-028 available from BYK Chemie

Wetting Agent

-   -   Dowanol® PM Glycol Ether available from the Dow Chemical Co.

Example 1

An aqueous mixture for forming an anode binder was formed. Contents ofthe aqueous mixture are described in the table below.

Aqueous Solid Cured Mixture Coating (wt. %) (wt. %) Additional Water18.33 0.00 PP 1 6.36 3.22 PP 2 6.36 3.22 PI 1 0.36 0.53 PI 2 0.07 0.11SEP1 61.45 88.73 Disp1 6.36 3.22 Accelerant 0.58 0.84 Photosensitizer0.11 0.16 TOTAL 100.00 100.00

The aqueous mixture contained 18.3 wt. % added water to assist with thedispersion of the graphite. The total water in the aqueous mixture was30.7 wt. % as many of the ingredients contained water. The coating wasapplied to a 25-um thick copper electrode, dried with a hot air gun andUV cured with 2 passes on a Miltec conveyor with a belt speed of 100ft/min and equipped with two Miltec HPI 650 W/in lamps. The coatingdensity was 8.3 mg/cm². The aqueous mixture could have been cured withan electron beam instead of UV light and the photoinitiators could thenbe deleted from the mixture.

The electrode and coating was evaluated in a coin cell test with lithiummetal as the counter electrode. Results are shown in FIG. 5A and FIG.5B. In the evaluation of FIG. 5A, the first discharge was at 48 mAh/gand the first charge was at 38 mAh/g. In the evaluation at FIG. 5B, thefirst discharge was at 313 mAh/g and the first charges was at 264 mAh/g.As shown, the lithium ion battery was able to charge and discharge.

Example 2

An aqueous mixture for forming an anode binder was formed. Contents ofthe aqueous mixture are described in the table below.

Aqueous Solid Cured Mixture Coating (wt. %) (wt. %) Additional Water18.71 0.00 PP 1 6.13 3.13 PP 2 6.13 3.13 PI 2 0.22 0.14 PI 3 0.23 0.32PI 4 0.22 0.31 SEP1 60.00 87.48 SEP2 1.70 2.48 Disp1 3.98 2.03Accelerant 0.52 0.76 Def1 0.19 0.22 Wetting Agent 1.99 0.00 TOTAL 100.00100.00

The aqueous mixture was applied to a 25-um thick copper electrode, usinga RK Control-Koter (R K Print-Coat Instruments Ltd., Hertz, UK) with a#9 Meyer rod. The water was dried from the coating with hot air. Thecoating was then UV cured with 2 passes on a Miltec conveyor with a beltspeed of 100 ft/min and equipped with two Miltec HPI 650 W/in lamps. Thecoating density was 7.59 mg/cm².

Results of examination are shown in FIG. 6A (Voltage change with respectto capacity) and FIG. 6B (Capacity change with respect to cycle number).This battery cycled through 40 cycles with increasing capacity as itcycled. The current density was 30 mA/g (C/10), the cut-off was0.001-1.5 V, the loading was 6.8 mg/cm² for the MCMB electrodeparticles. C/10 was utilized for 3 cycles and C/3 for the remainder. Ascan be seen, this battery had higher capacity that that of Example 1.This is believed to be due to the additional carbon black.

Example 3

An aqueous mixture for forming cathode binder was formed. Contents ofthe aqueous mixture are described in the table below.

Aqueous Solid Cured Mixture Coating (wt. %) (wt. %) Additional Water15.38 0.00 PP 1 3.75 1.64 PP 2 3.75 1.64 PI 1 0.37 0.46 PI 2 0.10 0.05SEP2 3.98 5.00 SEP3 69.33 87.00 Accelerant 1.63 2.05 Disp2 1.57 1.98Def2 0.15 0.18 TOTAL 100.00 100.00

While the foregoing written description enables one of ordinary skill tomake and use what is considered presently to be the best mode thereof,those of ordinary skill will understand and appreciate the existence ofvariations, combinations, and equivalents of the specific embodiment,method, and examples herein. The disclosure should therefore not belimited by the above described embodiments, methods, and examples, butby all embodiments and methods within the scope and spirit of thedisclosure as claimed.

What is claimed is:
 1. A method of forming an electrode comprising:applying an aqueous mixture to an electrode current collector to form alayer on the current collector, the aqueous mixture including a polymerprecursor, solid electrode particles, and water, the polymer precursorbeing dispersed or dissolved in the water; and subjecting the layer toelectron beam and/or actinic radiation thereby polymerizing andcrosslinking the polymer precursor to form an electrode binder thatbinds the solid electrode particles to the current collector and formthe electrode.
 2. The method of claim 1, the aqueous mixture furthercomprising a crosslinking agent.
 3. The method of claim 1, wherein theelectrode is a cathode.
 4. The method of claim 1, wherein the electrodeis an anode.
 5. The method of claim 1, the polymer precursor comprisinga monomer.
 6. The method of claim 1, the polymer precursor comprising anoligomer.
 7. The method of claim 6, wherein the oligomer is an oligomerof a polyurethane, a polyester, a polyolefin, a cellulose derivative, arubber, a latex, a polysulfide, an acrylic, or a silicone.
 8. The methodof claim 1, the aqueous mixture including the polymer precursor in anamount of from about 2 wt. % to about 25 wt. %.
 9. The method of claim1, the solid electrode particles including carbon particulate materials,silicon-based materials, metal oxide salts, ceramic materials, orcombinations thereof.
 10. The method of claim 1, the aqueous mixtureincluding the solid electrode particles in an amount of from about 50wt. % to about 90 wt. %.
 11. The method of claim 1, the aqueous mixturefurther comprising a photoinitiator.
 12. The method of claim 1, theaqueous mixture further comprising an additive.
 13. The method of claim1, wherein the aqueous mixture includes less than about 5 wt % by weightof the mixture organic solvents.
 14. The method of claim 1, wherein thelayer is formed according to a coating technique selected from gravure,flexo, slot die, reverse roll, knife over roll, flat and rotary screen,extrusion, or offset.
 15. The method of claim 1, wherein the layer isfrom about 1 micron to about 500 microns in thickness.
 16. The method ofclaim 1, further comprising applying an electrolyte adjacent to thelayer.
 17. The method of claim 16, further comprising including anelectrolyte separator in the electrolyte.
 18. The method of claim 1,further comprising assembling a second electrode adjacent to the firstelectrode.
 19. The method of claim 18, wherein the second electrode isidentical to the first electrode.
 20. The method of claim 19, furthercomprising assembling the first and second electrodes with anelectrolyte and a separator between the first and second electrodes. 21.The method of claim 1, the polymer precursors having a number averagemolecular weight of less than about 150,000.